Since 1821, Sir Michael Faraday adopted the magnetic force, generated by a current-carrying wire, to isolate and move continuously a magnetic pole in a coil. That was the first demonstration of an electric motor. In this kind of devices, the magnetic flux requires a continuous inversion to allow the rotation of the electrical motor.
Considering both micro and macro scale applications, so far current technologies in rotary motors domain are the following:                a) electrical motors and Micro-Electro-Mechanical-Motors (MEMS) based on the Faraday-Neumann-Lenz law, working on the interaction of magnetic fields and electric currents into conductor coils. The drawbacks regarding these devices are the construction limits in very small assemblies, restrained down to the millimetre scale, and the hazards rising from the use of high voltages and currents, always required in order to obtain useful torques;        b) reciprocating and rotating combustion engines, typically fed by non-renewable fuels, causing pollution of the environment and related troubles, dimensioned in the meter scale, and characterized by high operating temperatures and a number of hydraulic connections and moving mechanical components, thus reducing the reliability of the system;        c) Jet engines, e.g. gas turbines (such as in the form of Turboprop and Turboshaft) relying on Newton's 3rd Law of dynamics, and commonly based on Joule-Brayton thermodynamic cycle; they are typically known for the highest temperature reached during operation. Dimensions are comparable to the ones of the abovementioned engines, but the acoustic and environmental pollution troubles associated to them are much higher;        d) electromagnetic or chemical energy-driven molecular motors, able to perform rotary or linear motions;        e) Optical micro-motors, exploiting the radiation pressure deriving from the interaction between the light carried by an optical waveguide and a structure free to move.        
At present, the most challenging limitations for existing motors are a low deliverable power in the case of micro motors, or the substantial weight, size, and polluting emissions in the case of macro motors.
Radiation pressure generates optical forces inducing mechanical displacements in opto-mechanical systems. Cavity opto-mechanics principles are, however, the most efficient strategy to enhance the strength of the optical forces acting on the matter, obtained through improving the light-matter interaction occurring in resonant photonic systems. To date, the investigation of cavity-enhanced opto-mechanical systems is limited to linear displacement systems allowing unidirectional actuation.
US 2009/0116788 discloses controlling optical resonances between two spaced-apart, coupled strong-confinement photonic devices, wherein optical resonances are used to generate optically induced forces and achieve precise mechanical actuation in an opto-mechanical system made of the two coupled strong-confinement photonic devices. Axial approach or departure between two stacked photonic devices formed as ring resonators are disclosed.